Global illumination of a 3D scene in a media source by and large utilizes an algorithm, such as volumetric photon mapping, to diffuse inter-reflection within the scene. Generally, the algorithm is designed to take into effect the light ray coming from a source as well as light rays from the source reflected by other surfaces in the scene. Typically, images rendered using global illuminations tend to appear more realistic than images rendered using only a direct illumination algorithm.
Traditionally, volumetric photon mapping has been the only solution to render quality volumetric effects. Volumetric photon mapping is a general solution to volumetric rendering and is efficient in rendering both single scattering and multiple scattering with homogeneous and inhomogeneous media. However, volumetric photon mapping is computationally expensive if you specifically want to render single scattering with homogeneous participating media. For example, the visual effects of single scattering are typically high frequency and thus require dense sampling of light flux in order to render the light path properly. This dense sampling may lead to a substantial storage and computational cost, limiting both performance and the size of a scene that can be rendered.